1. Field of the Invention
The present invention relates to a kiln for baking ceramic materials, particularly a kiln of the type formed by a heated channel through which the ceramic materials to be baked are conveyed. These ceramic materials are conveyed inside the channel by means of conveyors. The conveyors can be constituted by carriages, which convey the ceramic material to be baked through the kiln, or preferably by a conveyor surface. The conveyor surface can be formed by a conveyor belt or preferably by a plurality of motorized rollers. Multiple burners are arranged along the heated channel so as to heat the internal volume of the kiln. The burners are generally arranged on the side walls of the channel that forms the kiln, both above and below the conveyor surface formed by the motorized rollers.
2. Description of the Prior Art
The burners used are of various kinds according to the requirements and in particular they have a radiant, convective, high- or low-speed flame. The general purpose is to achieve optimum transfer of the heat generated by the burner, so as to provide an environment which is as uniform as possible around the parts to be baked, without discontinuities with excessively hot or excessively cold regions or points.
It has been observed that burners with a high gas stream speed generally allow better mixing of the atmosphere inside the kiln, allowing to improve the uniformity of the temperature conditions even with kilns in which the channel has a particularly wide transverse cross-section. In this manner, the kinetic energy generated by the stream of burner gas in fact entrains more or less significant masses of the atmosphere inside the kiln, so as to perform a mixing action which strikes the product to be baked with significant speeds. The expression "burners with a high gas stream speed" is used with reference to burners with outlet speeds of more than 40 m/s, preferably more than 50 m/s, and up to as much as 200 m/s.
The above is the main field of industrial utilization of the invention but is not a limitation, since the kiln according to the invention can be used in any other equivalent field, as claimed.
Devices as described above are known but have some drawbacks. In particular, in the immediate vicinity of the outlet of the burners there is no mixing of the atmosphere inside the kiln, and accordingly there are considerable temperature differences. In other words, while the central region of the channel is heated adequately, with a sufficiently uniform temperature, the regions near the walls, where the burners are installed, have a high temperature gradient, generally with temperatures which are significantly colder than those of the central region.
This difference becomes increasingly significant as the outlet speed of the burnt gases rises and, of course, as the transverse cross-section of the kiln increases. Since burners with a high gas outlet speed are generally used with kilns which have a considerable transverse width, in such conditions the kiln cannot be utilized adequately, because only the central part of the roller conveyor surface can be loaded, while the lateral parts, near the walls, must be left free from ceramic material to be baked, in order to prevent the high temperature gradient that occurs in these regions from causing defective production, which must later be rejected. In practice, therefore, the use of kilns which have a considerable transverse width to increase production cannot achieve the intended purposes.
GB 2 099 120 discloses a kiln in which the outlet of the burners is not straight, but comprises a bent end, so that the stream of burner gas is not directed perpendicularly to the burner supporting wall, rather it is directed with an angle of about 40.degree. with respect to this wall. However, also in this case, the above problems are not solved. In fact, the hot gas stream, even if inclined of 40.degree., cannot be opened to a large diffusion angle and always gives rise to the above temperature differences, inside the kiln. Furthermore, it is impossible to adjust local temperature problems.
FR 811 785 discloses a kiln, that, in correspondence with the burners is so shaped that the hot gas stream is redirected also tangentially to the wall of the kiln. However this solution, developed for metallurgical kilns is not suitable for kilns for firing ceramic material. In fact, in our case, the material to be fired is much more sensitive to temperature differences. Furthermore this solution cannot be used with the above identified burners with a high gas stream speed. Finally it is impossible to carry out any adjustment.
DE 296 06 706 U1 discloses a kiln heated with burners operating with the so called Venturi effect. The outlet flow of these burners cannot be opened to a large diffusion angle. Furthermore the burners are considerably expensive.
DE 2 134 330 discloses a kiln, in which the end of the burners is so shaped that the hot gas stream is redirected tangentially to the wall of the kiln. However, the above problems cannot be solved. In fact, the hot gas stream, even if redirected tangentially, cannot be opened to a large diffusion angle and always gives rise to the above temperature differences, inside the kiln. Furthermore, it is impossible to adjust local temperature problems.
Belgian patent 537 014 discloses a kiln in which, in the fixed structure of the kiln, there is arranged a diffusion element for each burner. However, also in this case, it is impossible to adjust the firing effect of the burners according to the local requirements. Particularly it is impossible to test various heating conditions in a process of trial-error-correction.
U.S. Pat. No. 3,782,884 discloses details of a burner, but nothing that can be helpful to solve the above problems.
DE 3 807 495 and FR 2 197 456 disclose a kiln in which the hot gas stream cannot be opened to a large diffusion angle and gives rise to the above temperature differences. Also in this case, it is impossible to adjust the firing effect of the burners according to the local requirements. Particularly it is impossible to test various heating conditions in a process of trial-error-correction.